1. Field of the Invention
This invention relates to a color CRT (cathode-ray tube) displaying correction circuit and, more particularly, to a circuit for correcting CRT display brightness and image quality irregularities.
Further, the invention concerns, in a CRT having a relatively high resolution and required to provide satisfactory image quality such as a computer terminals, a CRT brightness irregularity correction circuit for correcting the brightness to obtain substantially uniform brightness over the entire screen, i.e., a brightness without reduction in edge portions of the screen compared to a central portion thereof.
Still further, the invention concerns, in a CRT as noted above, a CRT image quality correction circuit using a parabolic waveform voltage generation circuit for obtaining a substantially uniform image quality.
Yet further, the invention concerns, in a CRT as noted above, a CRT image quality irregularity correction circuit, which can correct reduced brightness, deteriorated focusing and increased electron beam spot diameter in edge portions of the screen compared to a central portion thereof, thus obtaining a substantially uniform image quality over the entire screen.
2. Description of the Related Prior Art
Presently used CRTs, either color or monochromatic, adopt an electron gun structure and ar called cathode modulation type. The brightness of the cathode modulation type CRT is substantially determined by the voltage applied between a cathode and a first grid. Therefore, for brightness control a video signal is applied to the cathode, and a DC voltage is applied to the first grid and usually varied.
Usually, the brightness is high in a central portion of the screen and low in edge portions thereof due to the CRT structure.
Recently, in computer terminal displays or the like cases of using multi-windows or the like are increasing with increase of the display dot number. If the CRT has brightness irregularities, the brightness is varied depending on the window display position, so that the screen can be seen very unsatisfactorily.
Japanese Patent Laid-Open Sho No. 50-110732, Utility Model Laid-Open Sho No. 61-16668 and Utility Model Laid-Open No. 62-89878 disclose techniques for solving the above problem. In these techniques, a correction circuit is provided for generating parabolic waveform signals by using integral circuits to integrate signals of deflection periods and superimposingly applying these parabolic waveform signals to the first grid of the CRT for increasing the brightness of the display on the screen as one goes toward the edges of the screen, thereby effecting the brightness irregularity correction over the entire screen.
In this prior art brightness irregularity correction circuit, the frequency of the parabolic waveform signals for the brightness irregularity correction is determined by the time constant of the circuit. Therefore, there is a first problem that if the circuit is used for a recently developed multi-scan type CRT which is capable of working frequency variation in a range of, for instance, 15.75 to 32 kHz and 48 to 64 kHz, it is necessary to set the time constant for every working frequency.
In addition, there is a second problem that since the signals are generated by the integral circuits, a time delay is produced to cause deviation from regular correspondence between the electron beam spot position on the screen and parabolic waveform voltage phase, so that satisfactory brightness irregularity correction can not be obtained.
The conventional correction circuits using the integral circuits for generating parabolic waveform signals, has a defect that a complicated adjustment is required corresponding to the changing of display contents, as shown in following table.
______________________________________ contents of change points to be adjusted ______________________________________ dimensions of amplitude of parabolic CRT screen waveform signals displaying position amplitude and phase of on CRT screen parabolic waveform signals CRT scanning amplitude and phase of frequency parabolic waveform signals, and time content of integral circuit ______________________________________
The cathode-modulation type electron gun of the CRT is known by various names depending on its structure. Basically, however, in the cathode modulation type anode current Ib in the CRT, i.e., the brightness on the screen, is varied depending on the voltage difference applied between first grid G1 and cathode K.
Likewise, the electron beam spot diameter on the phosphor screen and anode current Ib are varied depending on the voltage difference applied between first and second grids G1 and G2, and the focusing is varied depending on the voltage difference applied between first and third grids G1 and G3.
Presently, TV grade monitors are used with suitable DC bias voltages applied to first to third grids G1 to G3. Further, in monitors having comparatively high resolution, focusing errors at edge portions of the screen are avoided with a system called dynamic focusing, in which horizontal and/or vertical parabolic waveform voltages are applied to only third grid G3 for focusing control.
FIG. 1 is a schematic showing a prior art CRT dynamic focusing circuit for avoiding image quality irregularities.
Referring to the Figure, there are shown horizontal and vertical parabolic waveform voltage generators 1a and 2a, adding circuit 3a, amplifier circuit 4a, CRT 5, differential amplifier AMP3a', to which horizontal and vertical parabolic waveform voltages are supplied, feedback resistor R1 in the adding circuit for differential amplifier AMP3a', amplifier AMP4a' for amplifying the output of differential amplifier AMP3a' to a level of a drive voltage for third grid G3 of CRT 5, feedback resistor R2 for determining the gain of amplifier AMP4a' and variable resistor VR3a for controlling a parabolic waveform voltage necessary for third grid G3 of CRT 5. Coupling capacitor C3 is provided for coupling only the AC component of the horizontal and vertical parabolic waveform voltages as output signal of amplifier AMP4a' to third grid G3 of CRT 5 for compensation of focusing. There are further shown DC bias power supply E3 for third grid G3, output stabilization load resistor R4 for amplifier AMP4a' and bias resistor R5. Reference symbols G1, G2, K and H in the Figure designate first and second grids, a cathode and a heater of CRT 5.
Prior art CRT 5 has a relatively round and convex faceplate as shown by the solid line in FIG. 1. Recently, however, FS tubes, which have relatively flat and square faceplates as shown by the dashed line in FIG. 1, are being used extensively for better appearance of the screen. With a FS tube, the distance from the electron gun assembly to the phosphor screen varies greatly between the central and edge portions of the screen. Therefore, although a DC voltage application system (which is called static focusing in contrast to dynamic focusing) may be sufficiently be used for third grid G3 in the case of a FS tube CRT for household VT grading, with a computer terminal FS tube CRT having a high resolution compared to the TV grading it is necessary for obtaining a practically satisfactory display image quality to use a correction circuit for correcting focusing errors at edge portion of the screen, based on the dynamic focusing (in which a parabolic waveform voltage is superimposedly applied to only third grid G3).
Particularly, a computer terminal display or the like frequently uses multi-windows or the like when it is provided with an increased number of display dots. In this case, there is a peculiar problem that a window appearing on an edge portion of the screen has a less satisfactory appearance compared to a window appearing on the center of the screen due to image quality irregularities.
However, in the prior art displaying correction circuit the parabolic waveform signals for correcting the display quality irregularities has a fixed frequency determined by the time constant of the circuit. Therefore, a recently developed multi-scan type CRT which is capable of working frequency variation in a range of, for instance, 15.75 to 32 kHz or 48 to 64 kHz, has a fourth problem that it is necessary to set the time constant depending on the working frequency and a fifth problem stemming from the generation of signal with an integrating circuit that a regular correspondence between the beam position on the screen and parabolic waveform voltage can not be obtained due to a time delay. Therefore, satisfactory display quality correction can not be obtained. Furthermore, conventional displaying correction circuit also has a defect that a complicated adjustment is required, as shown in the above described table.
In the CRT, aside from the display quality correction focusing correction based on the dynamic focusing as noted above, so-called pin-cushion correction also using parabolic waveform voltages has been in practice.
FIG. 2 shows a prior art circuit structure for effecting the focusing correction and pin-cushion correction as noted above. Vertical deflection circuit 7, to which a vertical sync signal (V. SYNC) provides a vertical deflection output to vertical deflection coil V.sub.L fitted on the outside of CRT 5. Horizontal deflection circuit 8, to which a horizontal sync signal (H. SYNC) is supplied, provides a horizontal deflection output to a horizontal deflection coil H.sub.L also fitted on the outside of CRT 5. For the pin-cushion correction, first vertical parabolic waveform voltage generator 9 provides a first vertical parabolic waveform voltage according to the vertical deflection output for superimposition on a voltage in a predetermined part of horizontal deflection circuit 8. For the focusing correction, second vertical parabolic waveform voltage generator 10 generates a second vertical parabolic waveform voltage according to a vertical deflection component led out from a predetermined part of vertical deflection circuit 7. Further, horizontal parabolic waveform voltage generator 11 generates a horizontal parabolic waveform voltage according to the horizontal sync signal (H. SYNC). The second vertical parabolic waveform voltage and horizontal parabolic waveform voltage are superimposed on each other, and the resultant voltage is supplied to third grid (G3) modulator circuit 12 for superimposition on a high tension voltage component from high tension voltage generator 13 to produce a third grid modulation output which is supplied to third grid G3 of CRT 5.
The above circuit construction, however, requires two independent vertical parabolic waveform voltage generators 9 and 10, which is undesirable not only structurally but also economically. These problems are also encountered in case of CRT brightness irregularity correction by superimposing a DC bias voltage and a parabolic waveform voltage on each other, and applying the resulting signal to the first grid of the CRT.